code: def test_optical_density_manual():
      """Test optical density on known values."""
      test_tol = 0.01
      raw = read_raw_nirx(fname_nirx, preload=True)
      # log(1) = 0
      raw._data[4] = np.ones(145)
      # log(0.5)/-1 = 0.69
      # log(1.5)/-1 = -0.40
      test_data = np.tile([0.5, 1.5], 73)[:145]
      raw._data[5] = test_data
  
      od = optical_density(raw)
      assert_allclose(od.get_data([4]), 0.0)
      assert_allclose(
          od.get_data([5])[0, :2], [0.69, -0.4], atol=test_tol
      )
  
file_path: mne-python/mne/preprocessing/nirs/tests/test_optical_density.py
imports: import numpy
  import pytest
  from numpy.testing import assert_allclose
  from mne.datasets import testing
  from mne.datasets.testing import data_path
  from mne.io import BaseRaw, read_raw_nirx
  from mne.preprocessing.nirs import optical_density
  from mne.utils import _validate_type
label_desc: Use image node_id 3 for calling a global function with example usage: test_optical_density_manual() without return types